As is known, the techniques of micromachining semiconductor devices enable manufacture of micromechanical structures within layers, generally of semiconductor material, deposited (for example, a polycrystalline silicon layer) or grown (for example, an epitaxial layer) on sacrificial layers, which are then removed via etching.
For example, Micro-Electro-Mechanical System (MEMS) force or pressure sensor devices are known, comprising at least one flexible membrane integrated in or on a die of semiconductor material and whose bending, as caused by the action of external forces, is measured. The measurement may be of a piezoresistive type, and to this end piezoresistors are integrated in or on the membrane, or of a capacitive type, and to this end the membrane is capacitively coupled to another conductive region of the die. In either case, the variation of electrical signal resulting from deflection of the membrane is measured.
Microintegrated sensors generally have containers or packages for protecting the internal structures of the sensor from the external environment, for example in order to reduce disturbance due to temperature and humidity, and to the presence of particles which prevent their operation or worsen the performance thereof. The package also has the function of increasing the mechanical strength of the device.
In MEMS devices, manufacture of the package and its presence may cause stresses that adversely affect the performance, stability, and reliability of the sensor.
This is particularly disadvantageous for load sensors, which are, for example, based upon the piezoresistive characteristics of silicon, where the stresses are directly involved in the transduction mechanism. Thus, currently, in these cases, design of the device is aimed at limiting the effects of stress caused by the package and by the assembly process, for example with an accurate choice of the used materials and taking into account effects that arise during mechanical coupling between the sensor and the package.
The undesirable effects become increasingly important as the dimensions of the dice and of the packages increase and limit the use of 3D packaging techniques. For example, in pressure and force sensors, simple techniques of packaging by moulding, commonly employed in microelectronics, are not used since they generate high stresses during resin injection and cooling.
Further, undesirable deformations may arise also as a result of operating temperatures and material ageing.
In fact, the material of the package (typically plastic or metal) generally has a thermal expansion coefficient that differs considerably from that of the material of the structure (monocrystalline or polycrystalline silicon or ceramic).
The soldering processes or the temperature variations may thus bring about different deformations in the package and in the sensor element, which may cause thermo-mechanical stresses and strains (for example, according to the phenomenon known as “die warpage”), which cause measurement errors and drifts. These errors also vary according to the production lot, and at times even between sensors belonging to a same production lot, and are variable over time.
In order to eliminate these measurement errors, in the past a wide range of solutions have been proposed.
For example, various solutions of low-stress packages have been proposed and adopted. In some of these, the package also comprises mechanical structures for decoupling the sensor from the surrounding environment. However, these solutions do not solve the problem completely.
Other solutions envisage an electronic compensation of the thermal drifts of the measures supplied by the micromechanical structure via appropriate electronic components in the reading interface associated to the structure, for example an ASIC (Application-Specific Integrated Circuit).
A known solution of this type uses a temperature sensor in the reading electronics associated to the micromechanical structure. If the temperature is known, the drifts of the system are electronically compensated by resorting to compensation curves previously obtained via appropriate calibration and/or simulation procedures.
Solutions of this kind are, however, burdensome, in so far as they require costly and delicate measurement procedures to obtain compensation curves that accurately map the thermal drifts of the sensors, and appropriate compensation operations. Further, in general, the obtainable precision is not altogether satisfactory and repeatable.
Other proposed solutions thus provide for an integrated compensation of the thermo-mechanical deformations, by introducing structural compensation elements in the micromechanical structure. Also these solutions are unable to solve the problem satisfactorily.
There is a need in the art to provide a load sensor that overcomes the drawbacks of the prior art.